GR translation: grinding off

[Mike Jing]

V26.11-18, P26.33-39   Busses were grinding off, hundreds of them, down the
long concrete viaducts smeared with years’ pitiless use and no pleasure,
into haze-gray, grease-black, red lead and pale aluminum, between scrap
heaps that towered high as blocks of flats, down side-shoving curves into
roads clogged with Army convoys, other tall busses and canvas lorries,
bicycles and cars, everyone here with different destinations and
beginnings, all flowing, hitching now and then, . . .

What does "grinding off" mean here?